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Fuel Testing

Handling aircraft fuel must be done according to local airport safety regulations and rules. If not done properly the results can be really devastating to you and others. Aircraft fuel is highly combustible and burning AVgas is only useful to us inside an engine while trying to rotate the propeller. Here we are going to discuss safety practices everybody should adhere too.

Contaminated or the wrong type of fuel can be a recipe for an off airport landing, below you will find some procedures to test for water, alcohol and the fuel color.

Testing fuel

Fuel tester

Before the first flight of the day and after refueling (let the fuel settle first before testing so that any impurities will sink down) it is wise to see with a fuel tester if water or debris is in the tanks or engine gascolator. Do this with one of the samplers shown and drain the fuel tanks and fuel system at the lowest point. It is best to start at the tanks and work your way to the lowest point (usually the gascolator near the engine). This avoids water in the tanks being drawn into the fuel lines. See the POH for exact details of your aircraft.

Know how many drain points your aircraft has and after draining make sure the drain valve closes properly and does not keep on leaking fuel. And while you are at it: check the fuel tank vents for blockage.

Some have asked how water gets into a fuel tank. There are several reasons for that:

  • One source is the filler neck where fuel is added to a tank. Water from rain and from washing an aircraft can enter a tank through its filler neck if it's not properly sealed with a cap. Make sure to check the caps when refueling to check their condition.
  • Fuel can also be contaminated from condensation that occurs within the tanks. Over a period of time, condensation could introduce a hazardous quantity of water. To reduce condensation keep the tanks topped off when leaving the aircraft overnight. This way the amount of air (which can contain moisture) above the fuel level is kept to a minimum.
  • Water contamination can occur from a contaminated source during a refueling process. Fuel trucks and storage tanks are susceptible to water contamination. Therefore, the use of a water/debris separator is recommended when refueling.

Drained fuel

What to do with drained fuel that is clean? Well, I put it back in the fuel tank. You just concluded that fuel in the sample cup (which should be squeaky clean) is clean so the fuel in the tank must be too. Why not put it back then if then fuel in the sample cup is clean? If you will not trust the clean fuel in the cup why trust the fuel in the tank which you can not see anyway?

If it is contaminated, or if you are not sure about it, then put it in a separate container (clearly marked as contaminated) and use it for maybe a lawnmower or your car. Do not pour it on the platform or in the grass, if not sure, ask the FBO or airport operator what to do with drained fuel.
Another option is to save the drained fuel in a specially marked container and when its full pour it back in the aircraft through a special filter like Mr. Funnel.

Fuel color

As mentioned previously, aviation fuel is dyed with a specific color to distinguish the separate ratings. Below you can see which colors are used:

Fuel gradeColorUsageComments
AVgas 100/130GreenMost light piston engine aircraftWill cause spark plug fouling due to high lead content
AVgas 100LLBlueMost light piston engine aircraftLower lead version of above
AVgas 82 ULPurpleMost light piston engine aircraftNo lead version of above
JET A/A1Straw or clearTurbine/ Diesel aircraftDistinctive smell, not to be used in spark ignition engines
Diesel/BiodieselClearDiesel aircraftDistinctive smell, not to be used in spark ignition engines
AG DieselRedDiesel aircraftDistinctive smell, not to be used in spark ignition engines

When sampling, hold the container against a white background to verify the color, a sheet of white paper or against the wing (if its painted white) will do fine.

Testing for Alcohol in Mogas

The following steps describe how to test if there is any alcohol in your Mogas:

  • Using a glass or chemical resistant plastic (such as TPX) container, mark ten equally spaced volumes. A graduated cylinder is ideal; however, a non tapered glass jar, such as a large (quart) olive bottle, will work.
  • Add one part of water (approximately 100 ml) into the container, fill to the first mark, and then add nine parts (approximately 900 ml) of automotive gasoline, fill to the top mark. Shake thoroughly, let stand for ten (10) minutes or until automotive gasoline is again bright and clear. Record the apparent level of the line between the automotive gasoline and water.

Interpreting the above test:

  • If alcohol is present in the automotive gasoline, the water will absorb it, and the amount of water will appear to increase, indicating the automotive gasoline should not be used in the aircraft.
  • However, if the water level remains the same, then no alcohol is present in the automotive gasoline and it can be used safely.

Operators of aircraft approved for operation with Mogas containing methanol or ethanol shall consider the lower energy content of such a fuel (which will result in a lower performance or higher fuel consumption).

Written by EAI.

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